U.S. patent application number 14/267642 was filed with the patent office on 2014-11-06 for imaging of the eye.
This patent application is currently assigned to Optos PLC. The applicant listed for this patent is Optos PLC. Invention is credited to Jano Van Hemert, Michael Verhoek.
Application Number | 20140327877 14/267642 |
Document ID | / |
Family ID | 48627235 |
Filed Date | 2014-11-06 |
United States Patent
Application |
20140327877 |
Kind Code |
A1 |
Hemert; Jano Van ; et
al. |
November 6, 2014 |
IMAGING OF THE EYE
Abstract
A method of determining a geometrical measurement of a retina of
an eye, comprising obtaining a two dimensional representation of at
least a portion of the retina of the eye (34), deriving a
geometrical remapping which converts the two dimensional
representation of the retinal portion to a three dimensional
representation of the retinal portion (36), using one or more
coordinates of the two dimensional representation of the retinal
portion to define the geometrical measurement to be taken of the
retina on the two dimensional representation (38), using the
geometrical remapping to convert the or each coordinate of the two
dimensional representation of the retinal portion to an equivalent
coordinate of the three dimensional representation of the retinal
portion (40), and using the or each equivalent coordinate of the
three dimensional representation of the retinal portion to
determine the geometrical measurement of the retina of the eye
(42).
Inventors: |
Hemert; Jano Van;
(Edinburgh, GB) ; Verhoek; Michael; (Edinburgh,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Optos PLC |
Dunfermline |
|
GB |
|
|
Assignee: |
Optos PLC
Dunfermline
GB
|
Family ID: |
48627235 |
Appl. No.: |
14/267642 |
Filed: |
May 1, 2014 |
Current U.S.
Class: |
351/206 ;
351/246 |
Current CPC
Class: |
A61B 3/1025 20130101;
A61B 3/1005 20130101; A61B 3/12 20130101; G06T 2207/30041 20130101;
A61B 3/0025 20130101; G06T 7/0012 20130101; A61B 3/14 20130101;
G06T 7/64 20170101 |
Class at
Publication: |
351/206 ;
351/246 |
International
Class: |
A61B 3/14 20060101
A61B003/14 |
Foreign Application Data
Date |
Code |
Application Number |
May 2, 2013 |
GB |
1307990.0 |
Claims
1. A method of determining a geometrical measurement of a retina of
an eye, comprising (i) obtaining a two dimensional representation
of at least a portion of the retina of the eye, (ii) deriving a
geometrical remapping which converts the two dimensional
representation of the retinal portion to a three dimensional
representation of the retinal portion, (iii) using one or more
coordinates of the two dimensional representation of the retinal
portion to define the geometrical measurement to be taken of the
retina on the two dimensional representation, (iv) using the
geometrical remapping to convert the or each coordinate of the two
dimensional representation of the retinal portion to an equivalent
coordinate of the three dimensional representation of the retinal
portion, and (v) using the or each equivalent coordinate of the
three dimensional representation of the retinal portion to
determine the geometrical measurement of the retina of the eye.
2. A method according to claim 1 in which deriving the geometrical
remapping which converts the two dimensional representation of the
retinal portion to a three dimensional representation of the
retinal portion comprises determining a correction for distortion
introduced into the two dimensional representation of the retinal
portion in production thereof.
3. A method according to claim 2 in which distortion is introduced
into the two dimensional representation of the retinal portion by
optical and mechanical properties of an imaging device used to
produce the two dimensional representation and determining the
correction for distortion in the two dimensional representation
comprises modelling the optical and mechanical properties of the
imaging device.
4. A method according to claim 3 in which modelling the optical and
mechanical properties of the imaging device comprises (i)
constructing an optical description of an imaging system comprising
the imaging device and a model eye, (ii) passing a ray through the
imaging system onto a retinal surface of the model eye, (iii)
calculating an actual measurement of the ray at the retinal
surface, (iv) determining a horizontal scanning angle and a
vertical scanning angle of the imaging system for the ray, (v)
calculating an expected measurement of the ray at the retinal
surface using the horizontal scanning angle and vertical scanning
angle of the imaging system, (vi) repeating steps (ii) to (v) for a
plurality of further rays, and (vii) comparing the actual
measurements of the rays at the retinal surface with corresponding
expected measurements of the rays at the retinal surface to
determine the correction for distortion in eye representations of
the imaging device.
5. A method according to claim 4 in which determining the
horizontal scanning angle for a ray comprises ascertaining an
angular position of a horizontal scanning element of the imaging
device used to produce the ray and using this to calculate the
horizontal scanning angle for the ray.
6. A method according to claim 4 in which determining the vertical
scanning angle of the system for a ray comprises ascertaining an
angular position of a vertical scanning element of the imaging
device used to produce the ray and using this to calculate the
vertical scanning angle for the ray.
7. A method according to claim 3 further comprising determining the
correction for distortion in eye representations of the imaging
device for a plurality of non-zero gaze angles of the model
eye.
8. A method according to claim 2 in which determining the
correction for distortion in the two dimensional representation
comprises (i) creating a target, (ii) computing an image of the
target, (iii) using an imaging device used to produce the two
dimensional representation to scan the target to create a distorted
image of the target, and (iv) comparing the computed image of the
target with the distorted image of the target to determine the
correction for image distortion of the imaging device.
9. A method according to claim 1 in which the one or more
coordinates of the two dimensional representation of the retinal
portion used to define the geometrical measurement may define one
or more points on the two dimensional representation.
10. A method according to claim 9 in which using the geometrical
remapping to convert one or more coordinates of the two dimensional
representation of the retinal portion to an equivalent coordinate
of the three dimensional representation of the retinal portion
comprises using the geometrical remapping to map coordinates of one
or more points of the two dimensional representation into
coordinates of one or more equivalent points of the three
dimensional representation.
11. A method according to claim 1 in which the geometrical
measurement of the retinal portion comprises a distance measurement
of a structure of the retinal portion which is defined on the two
dimensional representation of the retinal portion using coordinates
of first and second points of the two dimensional
representation.
12. A method according to any claim 11 in which the geometrical
remapping is used to convert the first and second points of the two
dimensional representation of the retinal portion to equivalent
first and second points of the three dimensional representation of
the retinal portion and the equivalent first and second points of
the three dimensional representation are used to determine the
distance geometrical measurement.
13. A method according to claim 1 in which the geometrical
measurement of the retinal portion comprises an angle between first
and second structures of the retinal portion which is defined on
the two dimensional representation of the retinal portion using
coordinates of a first point and a second point of the two
dimensional representation to mark the first structure and using
coordinates of the first point and a third point of the two
dimensional representation to mark the second structure.
14. A method according to claim 13 in which the geometrical
remapping is used to convert the first, second and third points of
the two dimensional representation of the retinal portion to
equivalent first, second and third points of the three dimensional
representation of the retinal portion and the equivalent first,
second and third points of the three dimensional representation are
used to determine the angle geometrical measurement.
15. A method according to claim 1 in which the geometrical
measurement of the retinal portion comprises an area of a structure
of the retinal portion which is defined on the two dimensional
representation of the retinal portion using coordinates of a
plurality of points of the two dimensional representation.
16. A method according to claim 15 in which the geometrical
remapping is used to convert the plurality of points of the two
dimensional representation of the retinal portion to an equivalent
plurality of points of the three dimensional representation of the
retinal portion and the equivalent plurality of points of the three
dimensional representation are used to determine the area
geometrical measurement.
17. A computer readable non-transitory media storing program
instructions which, when executed, perform a method of determining
a geometrical measurement of a retina of an eye, comprising (i)
obtaining a two dimensional representation of at least a portion of
the retina of the eye, (ii) deriving a geometrical remapping which
converts the two dimensional representation of the retinal portion
to a three dimensional representation of the retinal portion, (iii)
using one or more coordinates of the two dimensional representation
of the retinal portion to define the geometrical measurement to be
taken of the retina on the two dimensional representation, (iv)
using the geometrical remapping to convert the or each coordinate
of the two dimensional representation of the retinal portion to an
equivalent coordinate of the three dimensional representation of
the retinal portion, and (v) using the or each equivalent
coordinate of the three dimensional representation of the retinal
portion to determine the geometrical measurement of the retina of
the eye.
18. An imaging system for determining geometrical measurements of
the retina of the eye, comprising an imaging device which obtains a
two dimensional representation of at least a portion of the retina
of the eye, a derivation element which derives a geometrical
remapping which converts the two dimensional representation of the
retinal portion to a three dimensional representation of the
retinal portion, a measurement definition element which uses one or
more coordinates of the two dimensional representation of the
retinal portion to define the geometrical measurement of the retina
on the two dimensional representation, a remapping element which
uses the geometrical remapping to convert the or each coordinate of
the two dimensional representation of the retinal portion to an
equivalent coordinate of the three dimensional representation of
the retinal portion, and a measurement determination element which
uses the or each equivalent coordinate of the three dimensional
representation of the retinal portion to determine the geometrical
measurement of the retina of the eye.
Description
RELATED APPLICATION
[0001] This application claims the benefit of and priority to UK
Application No. 1307990.0 filed on May 2, 2013, which is
incorporated herein by reference in its entirety for all
purposes.
FIELD OF THE DISCLOSURE
[0002] The present application relates to improvements in and
relating to imaging of the eye, particularly determining
geometrical measurements of the eye.
BACKGROUND
[0003] There are various types of imaging devices which may be used
to obtain images of an eye, for example ophthalmoscopes and fundus
cameras. Specifically, such devices take measurements of the retina
of the eye, which is a three dimensional structure approximated by
a sphere. The output of the imaging devices is a two dimensional
image of the three dimensional retina, as two dimensional images
have a number of advantages, not least that they are able to be
represented on a computer screen and output as photographs etc. The
imaging devices inspect, e.g. scan, the three dimensional retina
and produce two dimensional images of the retina. In inspecting the
three dimensional retina, the devices introduce distortion, which
distortion is determined by the optical and mechanical properties
of the devices. The distortion compromises one or more properties
of the two dimensional images of the retina, for example distance,
area and angle preservation. If geometrical measurements of a
retina are required, such as the distance between two structures of
the retina, and these are made using a two dimensional image
produced by such imaging devices, the results will be distorted and
inaccurate.
SUMMARY
[0004] According to a first aspect of the present solution there is
provided a method of determining a geometrical measurement of a
retina of an eye, comprising
(i) obtaining a two dimensional representation of at least a
portion of the retina of the eye, (ii) deriving a geometrical
remapping which converts the two dimensional representation of the
retinal portion to a three dimensional representation of the
retinal portion, (iii) using one or more coordinates of the two
dimensional representation of the retinal portion to define the
geometrical measurement to be taken of the retina on the two
dimensional representation, (iv) using the geometrical remapping to
convert the or each coordinate of the two dimensional
representation of the retinal portion to an equivalent coordinate
of the three dimensional representation of the retinal portion, and
(v) using the or each equivalent coordinate of the three
dimensional representation of the retinal portion to determine the
geometrical measurement of the retina of the eye.
[0005] Deriving the geometrical remapping which converts the two
dimensional representation of the retinal portion to a three
dimensional representation of the retinal portion may comprise
determining a correction for distortion introduced into the two
dimensional representation of the retinal portion in production
thereof. Distortion may be introduced into the two dimensional
representation of the retinal portion in production thereof by
optical and mechanical properties of an imaging device used to
produce the two dimensional representation.
[0006] Determining the correction for distortion in the two
dimensional representation may comprise modelling the optical and
mechanical properties of the imaging device.
[0007] Modelling the optical and mechanical properties of the
imaging device may comprise
(i) constructing an optical description of an imaging system
comprising the imaging device and a model eye, (ii) passing a ray
through the imaging system onto a retinal surface of the model eye,
(iii) calculating an actual measurement of the ray at the retinal
surface, (iv) determining a horizontal scanning angle and a
vertical scanning angle of the imaging system for the ray, (v)
calculating an expected measurement of the ray at the retinal
surface using the horizontal scanning angle and vertical scanning
angle of the imaging system, (vi) repeating steps (ii) to (v) for a
plurality of further rays, and (vii) comparing the actual
measurements of the rays at the retinal surface with corresponding
expected measurements of the rays at the retinal surface to
determine the correction for distortion in eye representations of
the imaging device.
[0008] Constructing an optical description of the imaging system
comprising the imaging device and the model eye may comprise
determining optical path properties of the imaging device,
determining optical path properties of the model eye and
concatenating the optical path properties to give optical path
properties of the imaging system. Determining the optical path
properties of the imaging device may comprise ascertaining
components of the imaging device which have an optical path effect,
ascertaining the order of the components in the imaging device,
establishing a mathematical function describing the optical path
properties over time of each component, concatenating in order the
optical path properties of the components to give the optical path
properties of the imaging device. Determining the optical path
properties of the model eye may comprise ascertaining components of
the model eye which have an optical path effect, ascertaining the
order of the components in the model eye, establishing a
mathematical function describing the optical path properties of
each component, concatenating in order the optical path properties
of the components to give the optical path properties of the model
eye.
[0009] Passing the rays through the imaging system onto the retinal
surface of the model eye may comprise using a ray tracing system.
The ray tracing system may be a commercially available ray tracing
system such as Zemax. Using the ray tracing system may comprise
loading the optical description of the imaging system into the ray
tracing system and determining a path through the imaging system
for each of the rays. Calculating the actual measurement of each
ray at the retinal surface may comprise calculating coordinates of
a point of intersection of each ray with the retinal surface.
[0010] Determining the horizontal scanning angle for a ray may
comprise ascertaining an angular position of a horizontal scanning
element of the imaging device used to produce the ray and using
this to calculate the horizontal scanning angle for the ray.
Determining the vertical scanning angle of the system for a ray may
comprise ascertaining an angular position of a vertical scanning
element of the imaging device used to produce the ray and using
this to calculate the vertical scanning angle for the ray.
[0011] Calculating the expected measurement of each ray at the
retinal surface may comprise using the horizontal scanning angle
and the vertical scanning angle of the imaging system to calculate
coordinates of a point of intersection of each ray with the retinal
surface.
[0012] Determining the correction for distortion in eye
representations of the imaging device may comprise deriving an
analytical transformation that maps the actual measurements of the
rays at the retinal surface of the model eye onto the corresponding
expected measurements of the rays at the retinal surface of the
model eye. Determining the correction for distortion in eye
representations of the imaging device may comprise constructing a
correction look up table (LUT) comprising, for each of a plurality
of rays, an actual location of the ray at the retinal surface of
the model eye against an expected location of the ray at the
retinal surface of the model eye.
[0013] The method of determining the correction for distortion in
the two dimensional representation may further comprise determining
the correction for distortion in eye representations of the imaging
device for a plurality of non-zero gaze angles of the model eye.
For each non-zero gaze angle, this may further comprise measuring
the gaze angle using a fovial location of the model eye. A
correction LUT may be constructed for each of the plurality of
non-zero gaze angles.
[0014] Determining the correction for distortion in the two
dimensional representation may comprise
(i) creating a target, (ii) computing an image of the target, (iii)
using an imaging device used to produce the two dimensional
representation to scan the target to create a distorted image of
the target, and (iv) comparing the computed image of the target
with the distorted image of the target to determine the correction
for image distortion of the imaging device.
[0015] Computing the image of the target may comprise calculating
coordinates of a plurality of points of the image of the target.
Scanning the target may comprise passing a plurality of rays
through the imaging device to determine coordinates of a plurality
of points of the distorted image of the target.
[0016] Comparing the computed image of the target with the
distorted image of the target may comprise comparing points of the
computed image of the target with corresponding points of the
distorted image of the target.
[0017] Determining the correction for image distortion of the
imaging device may comprise deriving an analytical transformation
that maps coordinates of points of the distorted image of the
target onto coordinates of corresponding points of the computed
image of the target. Determining the correction for image
distortion of the imaging device may comprise constructing a
correction look up table comprising, for each of a plurality of
possible points of the distorted image of the target, coordinates
of the possible point of the distorted image against coordinates of
the corresponding point of the computed image of the target.
[0018] The target may comprise a plurality of contrasting shapes.
The target may have a predefined pattern on it.
[0019] Determining the correction for distortion in the two
dimensional representation may comprise receiving an image of an
eye and transforming the image until it substantially registers
with an image of the eye produced by an imaging device for which
distortion correction is already known.
[0020] Determining the correction for distortion in the two
dimensional representation may comprise a third party determining
the correction and receiving the correction from the third
party.
[0021] The geometrical remapping which converts the two dimensional
representation of the retinal portion to a three dimensional
representation of the retinal portion may take the form of an
analytic equation that transforms coordinates of the two
dimensional representation into equivalent coordinates of the three
dimensional representation. The geometrical remapping which
converts the two dimensional representation of the retinal portion
to a three dimensional representation of the retinal portion may
take the form of a lookup table that lists coordinates of the two
dimensional representation equivalent to coordinates of the three
dimensional representation. The coordinates of the three
dimensional representation may be Cartesian coordinates or may be
spherical coordinates
[0022] The one or more coordinates of the two dimensional
representation of the retinal portion used to define the
geometrical measurement may define one or more points on the two
dimensional representation. The or each point on the two
dimensional representation may be identified using a Cartesian
coordinate space. The or each point on the two dimensional
representation may be identified on a screen using a pointing
device such as a mouse.
[0023] Using the geometrical remapping to convert one or more
coordinates of the two dimensional representation of the retinal
portion to an equivalent coordinate of the three dimensional
representation of the retinal portion may comprise using the
geometrical remapping to map coordinates of one or more point of
the two dimensional representation into coordinates of one or more
equivalent point of the three dimensional representation.
[0024] The or each equivalent coordinate of the three dimensional
representation of the retinal portion may comprise one or more
points on the three dimensional representation. The or each point
on the three dimensional representation may be identified using a
Cartesian coordinate space. The or each point on the three
dimensional representation may be identified using a spherical
coordinate space.
[0025] The geometrical measurement of the retinal portion may
comprise a distance measurement of a structure of the retinal
portion. The distance geometrical measurement may be defined on the
two dimensional representation of the retinal portion using
coordinates of first and second points of the two dimensional
representation. The geometrical remapping may be used to convert
the first and second points of the two dimensional representation
of the retinal portion to equivalent first and second points of the
three dimensional representation of the retinal portion. Using the
equivalent first and second points of the three dimensional
representation to determine the distance geometrical measurement
may comprise using the first point of the three dimensional
representation to define a start position of the measurement and
using the second point of the three dimensional representation to
define a finish position of the measurement and measuring the
distance between the first point and the second point of the three
dimensional representation. The three dimensional representation of
the retinal portion may be considered to be a sphere and the
distance between the first point and the second point measured as
the shortest distance between the first and second points of the
sphere.
[0026] The shortest distance between the first and second points on
the sphere may be measured in units relative to the sphere (degrees
or radians) using a spherical law of cosines method namely
.DELTA..sigma.=arccos(sin .phi..sub.s sin .phi.s.sub.f+cos
.phi..sub.s cos .phi..sub.f cos .DELTA..lamda.)
where .DELTA..sigma. is the central angle between the points,
.lamda..sub.s, .phi..sub.s, and .lamda..sub.f .phi..sub.f is the
longitude and latitude respectively of the first and second points
and .DELTA..lamda. is the absolute difference of the
longitudes.
[0027] The shortest distance between the first and second points on
the sphere measured in units relative to the sphere may be
converted into physical units (mm) using
d=r.DELTA..sigma.
where d is the distance between the points, r is the radius of the
sphere and .DELTA..sigma. is the central angle between the points
and is given in radians.
[0028] The shortest distance between the first and second points on
the sphere may be measured in units relative to the sphere (degrees
or radians) using Vincenty formulae namely
.DELTA..sigma. = arctan ( ( cos .phi. f sin .DELTA. .lamda. ) 2 + (
cos .phi. s sin .phi. f - sin .phi. s cos .phi. f cos .DELTA.
.lamda. ) 2 sin .phi. s sin .phi. f + cos .phi. s cos .phi. f cos
.DELTA..lamda. ) ##EQU00001##
where .DELTA..sigma. is the central angle between the points,
.lamda..sub.S .phi..sub.s and .lamda..sub.f .phi..sub.f is the
longitude and latitude respectively of the first and second points
and .DELTA..lamda. is the absolute difference of the
longitudes.
[0029] The shortest distance between the first and second points on
the sphere measured in units relative to the sphere may be
converted into physical units (mm) using
d=r.DELTA..sigma.
where d is the distance between the points, r is the radius of the
sphere and .DELTA..sigma. is the central angle between the points
and is given in radians.
[0030] The distance geometrical measurement may be output to a
user. The distance geometrical measurement may be represented on
the two dimensional representation of the retinal portion. The
shortest distance between the first and second points on the three
dimensional sphere (geodesic) will not necessarily correspond to a
straight line on the two dimensional representation of the retinal
portion. Representing the distance geometrical measurement on the
two dimensional representation may comprise parameterising the
distance between the first and second points on the sphere,
calculating a plurality of intermediate points and using the points
to represent the distance geometrical measurement on the two
dimensional representation.
[0031] The geometrical measurement of the retinal portion may
comprise an angle between first and second structures of the
retinal portion. The angle geometrical measurement may be defined
on the two dimensional representation of the retinal portion using
coordinates of a first point and a second point of the two
dimensional representation to mark the first structure and using
coordinates of the first point and a third point of the two
dimensional representation to mark the second structure. The
geometrical remapping may be used to convert the first, second and
third points of the two dimensional representation of the retinal
portion to equivalent first, second and third points of the three
dimensional representation of the retinal portion. Using the
equivalent points of the three dimensional representation to
determine the angle geometrical measurement may comprise using the
first point and the second point of the three dimensional
representation to mark the first structure and using the first
point and the third point of the three dimensional representation
to mark the second structure and measuring the angle between the
second point and the third point through the first point in the
three dimensional representation. The angle, C, may be measured
using the law of haversines, namely
haversin(c)=haversin(a-b)+sin(a)sin(b)haversin(C)
where haversin(x).ident.sin.sup.2(x/2), x1 and x2 are the first and
second points on the three dimensional representation which mark
the first structure, x1 and x3 are the points on the three
dimensional representation which mark the second structure, a is
the arclength of the geodesic between x1 and x2, b is the arclength
of the geodesic between x1 and x3 and c is the arclength of the
geodesic between x2 and x3.
[0032] The angle geometrical measurement may be output to a user.
The angle geometrical measurement may be represented on the two
dimensional representation of the retinal portion by representing
sides of the angle (the geodesic between x1 and x2 and the geodesic
between x1 and x3) on the two dimensional representation.
[0033] The geometrical measurement of the retinal portion may
comprise an area of a structure of the retinal portion. The area
geometrical measurement may be defined on the two dimensional
representation of the retinal portion using coordinates of a
plurality of points of the two dimensional representation. The
geometrical remapping may be used to convert the plurality of
points of the two dimensional representation of the retinal portion
to an equivalent plurality of points of the three dimensional
representation of the retinal portion. Using the equivalent
plurality of points of the three dimensional representation to
determine the area geometrical measurement may comprise using the
plurality of points of the three dimensional representation to
define a shape of the structure and measuring the area of the shape
in the three dimensional representation. The three dimensional
representation of the portion of the retina may be considered to be
a sphere and the shape of the structure may be defined as a
polygon, and the area of the polygon may be measured using
A = r 2 ( i = 1 n .alpha. i - ( n - 2 ) .pi. ) ##EQU00002##
where A is the area of the polygon, .alpha..sub.i for i=1, . . . ,
n are the n angles internal to the polygon and r is the radius of
the sphere. This yields a result in physical units (e.g. mm.sup.2
if r is given in mm). If r.sup.2 is omitted in the above formula, a
result is obtained in units relative to the sphere, in steradians
(sr), the unit of solid angle.
[0034] Using the equivalent plurality of points of the three
dimensional representation to determine the area geometrical
measurement may comprise using a plurality of pixels on the three
dimensional representation to define a shape of the structure and
measuring the area of the shape by summing the areas of the pixels.
The areas of the pixels on the three dimensional representation may
be calculated by assuming each pixel is a polygon with four points,
each point representing one corner of the pixel.
[0035] When the three dimensional representation of the portion of
the retina is considered to be a sphere, the radius of the sphere
can be determined by measuring the diameter of the eye. When the
three dimensional representation of the portion of the retina is
considered to be a sphere, the radius of the sphere can be set to
the average radius of the human eye, which is approximately 12 mm.
Even if the radius of the eye is not known or cannot be
approximated, relative measurements with respect to the size of the
sphere of the retina may be made by assuming the eye is a
unit-sphere. Distances on the sphere may be measured in radians or
degrees and areas in steradians or square degrees.
[0036] Obtaining the two dimensional representation of at least a
portion of the retina of the eye may comprise operating an imaging
device to produce the two dimensional representation. Obtaining the
two dimensional representation of at least a portion of the retina
of the eye may comprise receiving a two dimensional representation
already produced by an imaging device.
[0037] Obtaining the two dimensional representation of at least a
portion of the retina of the eye may comprise receiving a
distortion-corrected three dimensional representation of the
retinal portion and using a projection to obtain the two
dimensional representation from the three dimensional
representation. The projection may be chosen from a plurality of
known projections. The projection may be any of a conformal
projection, preserving angles in the two dimensional
representation, an equidistant projection, preserving distances in
the two dimensional representation, an area-preserving projection,
preserving areas in the two dimensional representation. Deriving
the geometrical remapping which converts the two dimensional
representation of the retinal portion to a three dimensional
representation of the retinal portion may comprise determining an
inverse of the projection used in obtaining the two dimensional
representation of the retinal portion.
[0038] According to a second aspect of the present solution there
is provided a computer readable non-transitory media storing
program instructions which, when executed, perform a method of
determining a geometrical measurement of a retina of an eye,
comprising
(i) obtaining a two dimensional representation of at least a
portion of the retina of the eye, (ii) deriving a geometrical
remapping which converts the two dimensional representation of the
retinal portion to a three dimensional representation of the
retinal portion, (iii) using one or more coordinates of the two
dimensional representation of the retinal portion to define the
geometrical measurement to be taken of the retina on the two
dimensional representation, (iv) using the geometrical remapping to
convert the or each coordinate of the two dimensional
representation of the retinal portion to an equivalent coordinate
of the three dimensional representation of the retinal portion, and
(v) using the or each equivalent coordinate of the three
dimensional representation of the retinal portion to determine the
geometrical measurement of the retina of the eye.
[0039] According to a third aspect of the present solution there is
provided an imaging system for determining geometrical measurements
of the retina of the eye, comprising [0040] an imaging device which
obtains a two dimensional representation of at least a portion of
the retina of the eye, [0041] a derivation element which derives a
geometrical remapping which converts the two dimensional
representation of the retinal portion to a three dimensional
representation of the retinal portion, [0042] a measurement
definition element which uses one or more coordinates of the two
dimensional representation of the retinal portion to define the
geometrical measurement of the retina on the two dimensional
representation, [0043] a remapping element which uses the
geometrical remapping to convert the or each coordinate of the two
dimensional representation of the retinal portion to an equivalent
coordinate of the three dimensional representation of the retinal
portion, and [0044] a measurement determination element which uses
the or each equivalent coordinate of the three dimensional
representation of the retinal portion to determine the geometrical
measurement of the retina of the eye.
[0045] The measurement definition element may be provided as
software or software and hardware in the imaging device. The
derivation element, the remapping element and the measurement
determination element may be provided as software in the imaging
device.
[0046] The same eye imaged on different conventional imaging
devices, will produce retinal representations that cannot directly
be compared nor be overlaid. This is also true for same-eye
representations produced by the same conventional imaging device,
but with different gaze angles. In an embodiment of the present
solution, once transformed into the three dimensional spherical
representation, it is possible to measure retinal structures in
physical units that are equivalent across imaging devices and
therefore allow direct comparison and derivations of relationships.
For example, measuring the distance between the centre of the optic
nerve head and the fovea on two images of the same eye produced by
different imaging devices should result in the same distance. Each
imaging device will have a different distortion due to different
optical and mechanical components as well as due to gaze angle
during imaging. By measuring the distance in the same spherical
representation it is possible to compensate for these
differences.
[0047] The geometrical measurements of the portion of the retina of
the eye may be used for: [0048] classification of disease/trauma
artefacts in the retinal portion, [0049] comparison of measurements
of structures of the retinal portion across different imaging
devices, [0050] longitudinal tracking of geometrical changes (e.g.,
changes in size, direction, distance) in anatomical structures and
pathologies in the retinal portion of the eye, [0051] development
of treatment planning based on the geometrical measurements, [0052]
direct delivery of treatment, such as photocoagulation, based on
geometrical coordinates derived from the geometrical measurements,
[0053] creation of normative databases based on distribution of
geometrical measurements of anatomical structures over
populations.
BRIEF DESCRIPTION OF DRAWINGS
[0054] Embodiments of the present solution will now be described by
way of example only, with reference to the accompanying drawings,
in which:
[0055] FIG. 1 is a schematic representation of an imaging system
according to the third aspect of the present solution, and
[0056] FIG. 2 is a flow chart illustrating the method according to
the first aspect of the present solution.
DETAILED DESCRIPTION
[0057] Referring to FIG. 1, the imaging system comprises an imaging
device 10, a derivation element 26, a measurement definition
element 28, a remapping element 30 and a measurement determination
element 32. In this embodiment, the measurement definition element
is provided as software and hardware, and the derivation element,
the geometrical remapping element and the measurement determination
element are provided as software in the imaging device.
[0058] The imaging device 10 comprises an ophthalmoscope. The
ophthalmoscope comprises a light source 12 emitting a beam of light
13, scan relay elements comprising a first scanning element 14, a
second scanning element 16, a scan compensation element 18 and a
scan transfer element 20. The first scanning element 14 comprises a
rotating polygon mirror and the second scanning element 16
comprises an oscillating plane mirror. The scan compensation
element 18 comprises an ellipsoidal mirror and the scan transfer
element 20 comprises an aspherical mirror.
[0059] The light source 12 directs an incident light beam 13 onto
the first scanning element 14. This produces a scan of the beam
(depicted by rays A, B and C) in a first, vertical, direction. The
incident beam impinges on the scan compensation element 18 and is
reflected from there onto the second scanning element 16. This
produces a scan of the incident beam in a second, horizontal,
direction. The incident beam then impinges on the scan transfer
element 20, which has two foci, the second scanning element 16 is
provided at a first focus and an eye 22 of a subject is provided at
the second focus. The incident beam from the second scanning
element 16 which impinges on the scan transfer element 20 will be
directed to the eye 22 and will impinge on a portion of the retina
of the eye. The light source 12 and the scan relay elements of the
ophthalmoscope 10 combine to provide a two dimensional scan of the
incident light beam 13 from an apparent point source and transfers
the two dimensional scan of the incident light beam from the
apparent point source to the retina of the eye. As the incident
light beam is scanned over the retina, it will be reflected
therefrom to produce a reflected light beam which is transmitted
back through the elements of the ophthalmoscope 10 and received by
one or more detectors (not shown). To acquire a representation of
the portion of the retina of the subject's eye 22, the incident
light beam from the source 12 is scanned over the retinal portion
in a raster scan pattern, produced by the first and second scanning
elements 14, 16 operating perpendicularly to each other and the
reflected light beam received by the one or more detectors.
[0060] Referring to FIGS. 1 and 2, the imaging device 10 obtains a
two dimensional representation of a portion of the three
dimensional retina of the eye 22 (step 34). The derivation element
26 receives the two dimensional representation of the retinal
portion and uses this to derive a geometrical remapping which
converts the two dimensional representation of the retinal portion
to a three dimensional representation of the retinal portion (step
36). Deriving the geometrical remapping comprises determining a
correction for distortion introduced into the two dimensional
representation of the retinal portion by optical and mechanical
properties of an imaging device used to produce the two dimensional
representation.
[0061] In a first embodiment, determining the correction for
distortion in the two dimensional representation comprises
modelling the optical and mechanical properties of the imaging
device 10. This comprises the following steps. An optical
description of an imaging system comprising the imaging device 10
and a model eye is first constructed. This comprises determining
optical path properties of the imaging device 10, determining
optical path properties of the model eye and concatenating the
optical path properties to give optical path properties of the
imaging system. Determining the optical path properties of the
imaging device 10 comprises ascertaining components of the imaging
device 10 which have an optical path effect, ascertaining the order
of the components in the imaging device 10, establishing a
mathematical function describing the optical path properties over
time of each component, concatenating in order the optical path
properties of the components to give the optical path properties of
the imaging device 10. Determining the optical path properties of
the model eye comprises ascertaining components of the model eye
which have an optical path effect, ascertaining the order of the
components in the model eye, establishing a mathematical function
describing the optical path properties of each component,
concatenating in order the optical path properties of the
components to give the optical path properties of the model
eye.
[0062] The optical description of the imaging system is loaded into
a ray tracing system, such as Zemax, a ray is passed through the
imaging system onto a retinal surface of the model eye and a path
through the imaging system for the ray is determined. An actual
measurement of the ray at the retinal surface is calculated as
coordinates of a point of intersection of the ray with the retinal
surface.
[0063] A horizontal scanning angle and a vertical scanning angle of
the imaging system for the ray is then determined. This comprises
ascertaining an angular position of a horizontal scanning element
of the imaging device used to produce the ray and using this to
calculate the horizontal scanning angle for the ray and
ascertaining an angular position of a vertical scanning element of
the imaging device used to produce the ray and using this to
calculate the vertical scanning angle for the ray.
[0064] An expected measurement of the ray at the retinal surface is
calculated using the horizontal scanning angle and vertical
scanning angle of the imaging system. This comprises using the
horizontal scanning angle and the vertical scanning angle of the
imaging system to calculate coordinates of a point of intersection
of the ray with the retinal surface.
[0065] The above steps are then repeated for a plurality of further
rays. The actual measurements of the rays at the retinal surface
are compared with corresponding expected measurements of the rays
at the retinal surface to determine the correction for distortion
in eye representations of the imaging device 10. The distortion
correction may take the form of an analytical transformation that
maps the actual measurements of the rays at the retinal surface of
the model eye onto the corresponding expected measurements of the
rays at the retinal surface of the model eye, or a correction look
up table (LUT) comprising, for each of a plurality of rays, an
actual location of the ray at the retinal surface of the model eye
against an expected location of the ray at the retinal surface of
the model eye. Determining the distortion correction may further
comprise determining the distortion correction for a plurality of
non-zero gaze angles of the model eye.
[0066] In a second embodiment, determining the correction for
distortion in the two dimensional representation comprises the
following steps. A target is firstly created comprising a plurality
of contrasting shapes. An image of the target is then computed. The
imaging device 10 used to produce the two dimensional
representation is used to scan the target to create a distorted
image of the target. Finally the computed image of the target is
compared with the distorted image of the target to determine the
correction for image distortion of the imaging device 10.
[0067] The distortion correction is then used to derive the
geometrical remapping which converts the two dimensional
representation of the retinal portion to the three dimensional
representation of the retinal portion. The geometrical remapping
takes the form of an analytic equation that maps coordinates of the
two dimensional representation into equivalent coordinates of the
three dimensional representation.
[0068] The two dimensional representation of the retinal portion is
accessed by the measurement definition element 28 and a distance
geometrical measurement is defined on the two dimensional
representation of the retinal portion using coordinates of first
and second points of the two dimensional representation (step 38).
This comprises displaying the two dimensional representation of the
retinal portion to a user, who identifies the first and second
points on the two dimensional representation using a pointing
device such as a mouse.
[0069] The remapping element 30 then uses the geometrical remapping
to convert the first and second points of the two dimensional
representation of the retinal portion to equivalent first and
second points of the three dimensional representation of the
retinal portion (step 40). This comprises using the transformation
to map coordinates of the points of the two dimensional
representation into coordinates of equivalent points of the three
dimensional representation. The points on the three dimensional
representation are identified using a spherical coordinate
space.
[0070] The measurement determination element 32 then uses the
equivalent first and second points of the three dimensional
representation to determine the distance geometrical measurement
(step 40). This comprises using the first point of the three
dimensional representation to define a start position of the
measurement and using the second point of the three dimensional
representation to define a finish position of the measurement and
measuring the distance between the first point and the second point
of the three dimensional representation, which is the length of the
structure of the retina of the eye 22. The three dimensional
representation of the retinal portion is considered to be a sphere
and the distance between the first point and the second point
measured as the shortest distance between the first and second
points of the sphere.
[0071] The shortest distance between the first and second points on
the sphere is measured in units relative to the sphere (degrees or
radians) using a spherical law of cosines method namely
.DELTA..sigma.=arccos(sin .phi..sub.s sin .phi..sub.f+cos.sub.s cos
.phi..sub.f cos .DELTA..lamda.)
where .DELTA..sigma. is the central angle between the points,
.lamda..sub.s .phi..sub.s and .lamda..sub.f .phi..sub.f is the
longitude and latitude respectively of the first and second points
and .DELTA..lamda. is the absolute difference of the
longitudes.
[0072] The shortest distance between the first and second points on
the sphere measured in units relative to the sphere may be
converted into physical units (mm) using
d=r.DELTA..sigma.
where d is the distance between the points, r is the radius of the
sphere and .DELTA..sigma. is the central angle between the points
and is given in radians.
[0073] The distance geometrical measurement is represented on the
two dimensional representation of the retinal portion, by
parameterising the distance between the first and second points on
the sphere, calculating a plurality of intermediate points and
projecting the points to represent the distance geometrical
measurement on the two dimensional representation. The distance
geometrical measurement represented on the two dimensional
representation of the retinal portion is then displayed to the
user.
* * * * *